A rucksack has two shoulder straps.

If it needs more explanation:
A rucksack has two straps which go across the carrier's shoulders.

- There may be two German expressions, but that doesn't force you to use two expressions in English.
- DO NOT translate "bzw." as "respectively" - that's a very bad mistake which will utterly perplex your reader.

maybe they mean 2 extra straps, i.e. a waist and a chest belt plus two shoulder straps as in http://www.gooutdoors.co.uk/expert-advice/ruc...
see common rucksack features, second picture

dann ist aber die deutsche Version nicht eindeutig..wenn es insgesamt 4 wären, hätte man doch UND geschrieben statt bzw.

@pittipal: point taken.
An "Umhängegurt" I'd imagine as a single long (handbag) shoulder strap, so I'm not sure how that would tie in with a rucksacky pair of shoulder straps anyway. And if it's meant to be the same, why mention it? Confusing.